Currently, ultrasound transducers of the type used in medical imaging mainly employ one-dimensional element arrays. Such an array comprises a linear grouping of transducer elements, wherein transmitted pulses are steered and dynamically focused only along the long dimension of the array. Transducers employing two dimensional element arrays are also known, however, the few two dimensional (2D) arrays that have been built to date are sector scanners. More particularly, the point of origin of a beam from such a 2D array is generally fixed in the center of the array and is scanned by appropriate phasing of the individual transducer elements of the array.
For certain types of imaging, e.g., abdominal imaging, linear arrays are typically used which translate the beam across the transducer face and thus form an image with a wide field of view at the skin surface.
Examples of two dimensional ultrasound arrays may be found in published PCT Application WO 94/21388 to Thomas et al. and U.S. Pat. No. 5,427,106 to Breimesser et al. A further example of a two dimensional ultrasound array is disclosed in U.S. Pat. No. 5,460,180 to Klepper et al. The Klepper et al. array comprises two different size transducer elements. Large transducer elements are positioned in the central portions of the transducer array, while more finely segmented transducer elements positioned at the ends of the array.
The Klepper et al. system is used to correct phase distortions of backscattered wavefronts which occur when the ultrasound signal propagates through an inhomogeneous medium, such as the human body. Such phase distortions occur as a result of variations in index of refraction through the tissues in the body. The effect of distortion in the elevation dimension is to produce a phase cancellation caused by a phase sensitive integration of the signal over the elevation dimension of the array elements. The approach used by Klepper et al. to correct for this phase distortion is to utilize the more finely segmented transducer elements, in combination with stepped groups of larger transducer elements, so as to enable the signals from the more finely segmented transducer elements to be utilized for the correction of the signals from the larger transducer elements. Such a correction scheme is implemented by sequentially multiplexing groups of the larger transducer elements across the face of the transducer along with a common connection of the finer transducer elements at the end of the transducer.
In substantially all of the linear, two dimensional transducer arrays of the prior art, the pitch between the transducer elements is generally maintained equal to or less than one half of the wavelength of the principal frequency emitted by the transducer. If the pitch distance between the elements is increased to greater than this value, acoustic beam side lobe signals are developed in the scan direction which tend to interfere with returning signals from the principal lobe direction. As a result, ultrasound units require large numbers of transducer elements that are closely spaced to avoid such side lobe projection in the scan dimension. In some cases, only a subset of the elements are used (known as a "sparse array"). This reduces element count but raises side lobe levels and causes a reduction in image quality.
There is a need for a two-dimensional wide area ultrasound scanning transducer that utilize a lesser number of transducers than are present in prior art arrangements and avoids side lobe generated signal anomalies.